Preheat your oven to 350°F (175°C). Grease the muffin tin or line it with muffin liners.
In a mixing bowl, mash the ripe bananas with a fork until smooth.
Add the creamy peanut butter, granulated sugar, eggs, and vanilla extract to the bowl. Mix well using a whisk until all the ingredients are fully combined.
In a separate bowl, whisk together the all-purpose flour, baking soda, salt, and cinnamon (if using).
Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Be careful not to overmix.
If desired, fold in the chopped walnuts or chocolate chips for added flavor and texture.
Evenly distribute the batter into the prepared muffin tin, filling each cup about two-thirds full.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and allow them to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.